Passively Q-switched microchip lasers (MCLs) have been demonstrated as a promising concept for realizing compact laser-sources for various applications. MCLs employing neodymium doped yttrium vanadate (Nd:YVO4) as a solid-state gain medium and passively Q switched by a semiconductor saturable absorber mirror (SESAM) can generate pulses with durations in the 100 picosecond (ps) range with energies of several 100 nanojoules (nJ). One microchip laser suitable for use with this gain medium is described in detail in U.S. Pre-grant Publication No. 20110243158, the complete disclosure of which is hereby incorporated herein by reference.
Such MCLs have inherently single longitudinal mode operation, and exhibit nearly diffraction-limited beam quality. This makes these MCLs potentially suitable for applications including spectroscopy, frequency conversion, micromachining, light detection and ranging (LIDAR), and precision medical and dental operations. Where additional power is required, the output of an MCL can be optically amplified creating a MOPA system with MCL as master oscillator. One optical amplifier suitable for this purpose is described in U.S. Pat. No. 7,256,931. This amplifier is a compact, multi-pass, grazing-incidence amplifier employing a thin, relatively short slab of Nd:YVO4, faced-pumped by a diode-laser array (diode-laser bar). The compact nature of the amplifier, in conjunction with the compact MCL provides for a correspondingly compact MOPA system.
A further reduction of the pulse duration, for example, from the 100 picoseconds or so provided by the MCL, to 50 ps or less, would increase the number of industrial and scientific applications for such MOPA systems. A paper by R. Lehneis et al., OPTICS LETTERS Vol. 37, No 21, 4401-3, describes shortening the duration of pulses from a Nd:YVO4 MCL by spectral broadening and simultaneously amplifying the pulses in a fiber pre-amplifier (without increasing the pulse-duration) then spectrally filtering the spectrally-broadened pulses to yield pulses having a duration about one-third of the duration of the pulses from the MCL. Spectral filtering was accomplished using a volume Bragg grating (VBG). It is taught that for optimal temporal quality of the shortened pulses, the spectral selection must be made from an edge of the broadened spectrum.
While the method of Lehneis et al. is elegant as an academic demonstration, it has certain shortcomings as far as realizing a commercial laser product is concerned. The requirement for a VBG would at least add significant cost to the system. This could possibly be somewhat mitigated by using a Lyot filter of comparable efficiency and sufficiently narrow bandwidth, for example, less than about 1 nanometer. However, such a filter may be subject at least to temporal drift of center wavelength of the pass-band of the filter. A drift toward the center of the broadened pulse spectrum would result in reduced temporal quality of the shortened pulses. A drift away from the center of the broadened spectrum could result in at least a significant reduction of power in the shortened pulses. There is a need for a more cost effective and reliable method of shortening pulses for a MCL.